To verify a Double Data Rate (DDR) interface, numerous out-of-chip screenings may be required to ensure compliance with manufacturer's specifications. Such out-of-chip screenings may be performed by automatic test equipment (ATE), for example. An ATE may apply functional test patterns to the chip containing the DDR interface. The result from applying a test pattern may be collected at the DDR interface output. Another set of patterns may be applied back by the ATE, whose end result may be collected at some other interface. A test pattern may be selected so that if the DDR interface functions properly and is compliant with specifications, a known result may be expected after the ATE verification process is complete.
However, problems with out-of-chip screenings may arise especially at higher operating frequencies, for example if a DDR interface is operating at 200 MHz (i.e., the data rate is at 400 MHz). A vector slip may occur, which does not allow for a proper screening. For example, a sequence of test vectors may be applied to the DDR interface at a time tn and a specific pattern output, or output vector, may be expected at the DDR interface output at a later time tn+m. However, because of process variation within the DDR interface, a vector slip may occur and the expected output vector may be generated at the DDR interface output at a time tn+m+2, i.e., two clock cycles later than expected. In addition, during out-of-chip screening, the DDR interface does not utilize the on-chip PLL clock for the screening process since an out-of-chip process is applied and there is no synchronization between the internal DDR interface PLL clock and the external clock related to the generated out-of-chip testing vectors. An out-of-chip clock, therefore, has to be utilized to accomplish proper DDR interface screening. An out-of-chip clock, however, may not account for process variations (e.g., jitters) within the DDR interface.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.